The neonatal hyperbilirubinemia study has been designed to assess the relationship of intermediate levels of serum bilirubin on the subsequent neurological and mental development of Collaborative Perinatal Project children. There has been increasing concern that neonatal serum bilirubin level between 10-20 mg. percentage may be damaging to the central nervous system; not in the classical sense of "kernicterus" associated with levels above 20 mg. percentage, but rather damaging in more subtle yet clinically significant ways. Neonates are to be studied in five birthweight-gestational age categories, by three socio-economic classes, for a variety of outcome measures; including mental and motor assessments of 8 months of age, and a spectrum of neurological findings at age one year, which will include motor performance, reflexes, tone, abnormal movements, eye findings, and the over-all neurological classification of normal, suspect, or abnormal. Plans call for submission of a paper for publication near the end of the current fiscal year.